OEIGIN OF THE CORPUS CALLOSUM. 



63 



plane in which the dorsal limb already lies. Xow this plane is directed backwards and 

 upwards, as indicated by an arrow in fig. 1. 



What are the effects of sucli a backward extension upon the hippocampus ? The 



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F/c.Z 



F/a 



Fig. 1. — Scheme to show the arrangement of the hippocampus in the mesial wall of the hemisphere of a Marsupial. 



The darkly-shaded part represents the first corpus eallosum, such as is found in Miiuojjtenis, and the arrow 



indicates the Une of its backward extension in the higher Mammals. 

 Fig. 2. — Scheme of the corresponding region of a more highly developed cerebrum, such as that of Pteropus. 

 Fig. 3. — The " spherical " region from fig. 2 on a larger scale. 



remains of the hippocampus which cover the dorsal aspect of the corpus eallosum are 

 stretched, and thus further attenuated, by the lengthening commissure. The backward 

 growth of the commissure, owing to its obliquity, soon indents the hippocampal 

 formation just at the junction of the atrophic " supracommissural " segment with the 



SECOND SERIES. — ZOOLOGY, VOL. VII. 10 



